1. Field of the Invention
The present invention relates to a sound acquisition method and system. The invention also relates to a sound acquisition and reproduction apparatus employing this method.
2. Description of the Related Art
The present invention has a main application in the field of audioconferencing, in which a sound acquisition and reproduction device is comprised in a single assembly of relatively small dimensions. This assembly must be able to be stood easily on a table and operate in any room without the necessity for acoustic treatment of these premises. It is desirable that it can be used by a person having great freedom of movement within a radius of at least 4 m around the device, while carrying on the conversation with his correspondent in normal comfortable listening conditions for the two correspondents.
Preferably, it may also be used by any number of persons assembled in the same premises and distributed around the item of furniture on which the device is stood. In order to obtain these results, four conditions are sought:
1. The device must be associated with two automatic level regulators which ensure that the correct level of a signal is sent to line, whatever the acoustic power gathered by the microphone(s) of the device, depending on the position of the speaker(s) with respect to this microphone or these microphones, and that the correct level of signal is sent to the loudspeaker(s), whatever the attenuation applied by the line.
2. The sound reproduced by the loudspeaker(s) must be perceived with sufficient listening comfort independently of the position occupied by the listener(s) in the premises.
3. The sound gathered by the microphone(s) must keep sufficiently stable qualities of clarity, of cleanliness and be pleasant to listen to, whatever the position of the speaker(s) with respect to the device, and whatever the configuration of the premises.
4. The device must exhibit good acoustic decoupling between the loudspeaker(s) and the microphone(s) so as to be able to ensure a sufficiently high sound listening level without causing the LARSEN effect, but also in order to send the least possible acoustic echo to the distant correspondent.
Operational devices satisfying condition 1 are currently known.
For example, devices exist which favor condition 4 by using a single microphone and four loudspeakers oriented along four directions spaced by an angle of 90.degree. from one another, and driven in phase opposition in pairs. This method makes it possible effectively to obtain low coupling since the microphone is placed at a point which is a center of symmetry with respect to the loudspeakers. As the latter are driven in phase opposition in pairs, and providing that they have identical characteristics, the sound originating from the loudspeakers gathered by the microphone will be very weak and thus the decoupling will be very good.
However, this type of device badly fulfills conditions 2 (because of the phase shifts of 180.degree. between loudspeakers, the radiation diagram of the set of loudspeakers will not be circular in the horizontal plane and will depend strongly on the frequencies emitted) and 3 (since the microphone picks up the direct sounds and the indirect, reflected sounds indifferently, which means that the quality of the sound picked up by the microphone depends too greatly on the position of the speaker in the premises and on the configuration of these premises).